Ceramic body for spark plug insulators



place, the greater the number of vesicles.

Patented Oct. 31, 1939 UNITED STATE CERALHC INSULATORS Taine G.McDougal,

Blair Barlett, Flint, Motors Corporation, tion of Delaware No Drawing.

Application Albra H. Fessler, and Helen Mich,

assignors to General Detroit, Mich., a corpora- September '1, 1938,

Serial No. 228,820

2 Claims.

This is a continuation in part of our prior application Serial No.50,243 filed November 16,

This invention has to do with ceramic bodies especially adapted for useas spark plug insulators. We have found that compositions consistingprincipally of aluminum oxide and tantalum oxide when finely ground,formed into bodies and sintered at high temperatures are converted intoa non-porous ceramic product of such dense and compact structure as toresult in an. improvement in certain desirable physical characteristics,such as mechanical strength, thermal efficiency, electrical resistanceat elevated temperatures, and resistance to heat shock. When aluminarecrystallizes it has a tendency to contain gas vesicles or blebs,especially in the center of the individual crystals, even though it hasbeen completely melted and allowed to crystallize from the molten state.The lower the temperature at which this recrystallization takes Normallycorundum crystals are nearly opaque as a result of this condition. Thephysical properties of completely clear crystals vary rather widely fromthose of the highly vesicular crystals which contain an appreciablevolume of voids. We have found it possible to reduce these vesicles andto obtain homogeneous crystals by the addition of tantalum oxide toalumina. Further, the addition of this material induces formation ofthis denser and more compact structure at greatly reduced temperatures,which is economically highly desirable.

Study of our improved bodies indicates that tantalum oxide formseutectics with alumina. The final product is a mechanical mixture of theoriginal components, i. e., the added component does not enter thelattice structure of the corundum and the physical properties are theresultant of the physical properties of the crystal phases which composethe mixture. As tantalum oxide is added to alumina in increasingquantities the properties of the insulator become more like those oftantala and less like those of alumina, always, of course, taking intoaccount the effects each component has on the crystal habit of theother, these effects in the case of tantala including the important onepreviously mentioned of increasing the density of the alumina crystals.

Best results have been obtained with a tantalum oxide content of from.5% to 30%. Such bodies are characterized by unusually high thermalefficiency. We have also been successful in making good bodies with apreponderance of tantalum oxide, for example as much as However, forcommercial production a tantalum oxide content of about 3% will probablybe found most satisfactory, thereby obtaining substantial improvement atlow cost.

The improved insulators are characterized by high thermal efflciency,good thermal shock resistance and high mechanical strength. They possesssatisfactory electrical resistance at high temperature and altogetherpossess the qualities necessary for successful operation under severeoperating conditions, particularly those encountered in modern highcompression engines.

The improved insulators are manufactured by first grinding andthoroughly mixing the ingredients. The grinding is preferably carried toi a point where all of the material is in the form of a fine powdercapable of passing through screens of 325 mesh to the linear inch. Theingredients should be substantially free from alkali 20 because of thedeleterious effect of the latter on the electrical properties of theinsulator. The insulators may be molded into shape with the aid of asuitable binding agent by the process described and claimed in PatentNo. 2,122,960 25 granted to Karl Schwartzwalder on July 5, 1938;- bypressing in rubber molds as described in Patent No. 2,091,973 granted toAlbra H. Fessler and Ralston Russel, Jr., on September "I, 1937; bycasting, or by any other of the known methods used in forming bodiesfrom non-plastic materials. The first mentioned method has been found tobe preferable because the resulting insulator usually possesses asmooth, glossy surface requiring no glaze.

The formed bodies are then fired to a suitable temperature to producerecrystallization, and this is usually accompanied by considerableshrinkage. The firing time and temperature must be controlled as inusual kiln practice so as to secure 7 complete recrystallization and toavoid overfiring with resultant warpage or other injury to the product.Firing temperatures on the order of Orton Cone 30 (1650 C.) will befound satis- 45 factory.

It may, in some instances, prove desirable to add to the compositionsmall proportions, for example up to 5%, of a suitable flux such as talcor fluorspar to reduce the firing tempera- 50 ture. It may also provedesirable to add proportions of other eutectic forming materials such ascerium oxide, monazite sand, zirconium oxide or thorium oxide; orproportions of inorganic compounds forming solid solutions with alumina55 such as those described in copending applications Serial Nos. 134,124and 134,125, filed by Taine G. McDougal, Albra H. Fessler and KarlSchwartz- We claim:

10 l. A spark plug insulator in the form of a dense,

nonporous body consisting of a sintered mixture of corundum and from .5%to 80% tantalum oxide.

2. A spark plug insulator in the form of a dense, nonporous bodyconsisting of a sintered mixture of corundum and from 5% to 30% tantalumoxide.

TAINE G. McDOUGAL. ALBRA H. FESSLER. HELEN BLAIR BARLE'I'I'.

